Poly-resistors are extensively used in integrated semiconductor chips. Poly-resistors are characterized by their sheet resistance values. In an effort to reduce the chip size, poly-resistors with high sheet resistance values are being fabricated in a small area. FIG. 1 schematically shows an example of a conventional poly-resistor 102 of the prior-art. Conventional poly-resistor 102 comprises a substrate layer 104, a dielectric layer 106 and a resistive strip 108. Resistor contacts 110 are provided on either sides of resistive strip 108. Resistive strip 108 is a poly-silicon strip. When a voltage is applied across resistor contacts 110 and substrate layer 104, depletion or accumulation of charge carriers occurs in the bottom portion of resistive strip 108 and upper portion of substrate layer 104, as shown by 110a and 110b, respectively. The depletion or accumulation of charge carriers depends upon the type of voltage applied. Thus, a parasitic MOS capacitance is formed across resistive strip 108 and substrate layer 104 due to the accumulation or depletion of the charge carriers. Hence, the conductivity of resistive strip 108 changes. In other words, the resistivity of resistive strip 108 changes. This effect is known as poly-depletion effect.
The poly-depletion effect is prominent in case of poly-resistors with high sheet resistance values. When a voltage is applied to a poly-resistor with high sheet resistance the poly-region gets depleted easily. Hence, its resistivity changes rapidly with a change in the voltage and such a poly-resistor exhibits a highly non-linear voltage-current behaviour. Further, when a conventional poly-resistor 102 is used in an amplifier circuit, the behaviour of the circuit is affected due to the poly-depletion effect. FIG. 2 schematically shows an example of an embodiment of an amplifier circuit 200, of the prior-art. Amplifier circuit 200 is an inverting amplifier circuit. Amplifier circuit 200 comprises operational amplifier 202, input conventional poly-resistor 102a and feedback conventional poly-resistor 102b. Input poly-resistor 102a is connected between input signal stage, Vin, and inverting terminal 204 of operational amplifier 202. Feedback poly-resistor 102b is connected between output terminal 208 and inverting terminal 204, at junction point 210. Non-inverting terminal 206 is either grounded or supplied with a fixed voltage.
As the gain of the amplifier circuit 200 is determined by the ratio of the resistances of input conventional poly-resistor 102a and feedback conventional poly-resistor 102b, the poly-depletion effect causes a non-constant gain of the amplifier circuit 200 when the input voltage Vin changes. Hence, amplifier circuit 200 amplifies input signal Vin with a large amount of distortion which degrades the quality of the output signal.